1. Field of the Invention
The present invention relates to the shutter of a camera, and especially to a shutter wherein a plurality of shutter blades are driven by an electromagnetic force.
2. Description of the Prior Art
Heretofore, shutters have been made so that shutter blades which also operate as diaphragm blades are electromagnetically driven to gradually open an aperture, and so that when a predetermined amount of exposure is obtained, the shutter blades are quickly closed. Such a device is known, for example, from Japanese Laid-Open Patent Applications No. Sho 55-57832 and No. Sho 55-101924. However, this type of shutter has heretofore been provided with a coil at a sector ring to drive the shutter blades, and the sector ring is directly driven by the coil with an electromagnetic driving force generated within a magnetic field as power supplied thereto to open the shutter blades. Therefore, the force to open the shutter blades varies with the value of current supplied to the coil. Since copper wire used in the coil will vary in its resistance by about 0.4%/.degree.C. depending upon the Joules of heat supplied thereto from a power source or due to a change in ambient temperature, the electromagnetically driven shutter develops shortcomings in that the driving force to open the shutter blades often varies causing adverse effects. Thus, the shutter opening characteristic cannot be maintained constant. Thus, it becomes difficult to always obtain the appropriate exposure.